The cranial nerves II, III, IV Va and VI contribute to functions of the eye. Besides the optic nerve, the rest travel through the cavernous sinus and superior orbital fissure. CN II, IV and VI which provide motor control of the eye are often tested by having a patient tracing the letter “H” with their gaze.

LR6SO4 (CN VI innervates lateral rectus, CN IV innervates superior oblique m.)

Mnemonic (innervation of eye muscles)

CN II (Optic nerve)

The optic nerves are purely sensory, with fibres made from the axons of retinal ganglion cells that (mostly) terminate at the lateral geniculate nucleus. As outgrowths from the CNS, they are covered by the meninges and pass through the optic canal into the cranium. Because there is a subarachnoid space, increased intracranial pressure can be transmitted via cerebrospinal fluid causing the optic disc to bulge (papilloedema). Above the sella turica, the left and right nerves undergo partial decussation at the optic chiasm so that left field vision travels to the right hemisphere and vice-versa.

CLINICAL CORNER

Bitemporal hemianopia, where lateral vision in both eyes is lost, can be caused by a pituitary adenoma impinging on the optic chiasm.

CN III (Oculomotor nerve)

The oculomotor nerve exits the oculomotor (motor) and Edinger-Westphal (parasympathetic) nuclei in the midbrain and pierces the roof of the cavernous sinus, running in its lateral wall. It divides into superior and inferior divisions before exiting the sinus, both of which pass through the superior orbital fissure and tendinous ring. The superior division supplies the levator palpebrae superioris and superior rectus. The inferior division supplies the inferior rectus, medial rectus and inferior oblique muscles. The inferior division also carries parasympathetic supply to the ciliary muscle (changes lens shape) and sphincter pupillae via the ciliary ganglion (between the optic nerve and lateral rectus) that gives off 10-12 short ciliary nerves.

CLINICAL CORNER

An oculomotor nerve lesion could paralyse most of the extraocular muscles, resulting in complete ptosis (drooping eyelid) and an immobile eyeball with a downwards and outwards gaze. As each eye points in different directions, the patient will experience diplopia (double vision). If CN III parasympathetic supply is disrupted, the pupil is dilated and unresponsive to light or when the eye focuses on a close object (accommodation) because of paralysis to the constrictor pupillae and ciliary muscles.

CN IV (Trochlear nerve)

The trochlear nerve arises from the trochlear nucleus and decussates before exiting the dorsal aspect of the brainstem at the caudal mesencephalon level. Then, it enters the cavernous sinus and runs forwards to enter the orbit through the superior orbital fissure, turning medially above levator palpebrae superioris to supply superior oblique. Superior oblique actions include depression and intorsion of the eyeball.

CLINICAL CORNER

A trochlear nerve lesion can cause vertical and torsional diplopia (double vision) when looking down or tilting the head. It is tested by looking medially and downwards – when the eyeball is maximally adducted, the superior oblique should depress the eye as it inserts laterally and posterior to the equator of the eyeball.

CN Va (Opthalmic nerve)

The ophthalmic nerve is the first of three branches of the trigeminal nerve (CN V). It passes into the cavernous sinus, where it picks up postganglionic sympathetic fibres from the internal carotid plexus and goes on to supply the vessels of the orbit, vessels and sweat glands of the forehead, and dilator pupillae. Its three main branches include the frontal, lacrimal, and nasociliary nerves.

*More information found on trigeminal nerve post

CN VI (Abducens nerve)

The abducens nucleus in the pons sends out fibres that travel in the subarachnoid space and pierce the dura on the posterior aspect of the clivus. It then travels superolaterally to enter the cavernous sinus, lying just lateral to the internal carotid artery. It leaves the sinus to pass through the superior orbital fissure, supplying lateral rectus.

CLINICAL CORNER

The abducens nerve is the most frequently damaged of the nerves to the eye as it is very sensitive to raised intracranial pressure and can be compressed by a cavernous sinus aneurysm or skull base brain tumour. Damage to the peripheral part of one nerve (such as in Sixth nerve palsy) causes diplopia (double vision), while damage to the abducens nucleus causes a horizontal gaze palsy affecting both eyes as the nucleus communicates with the oculomotor nucleus to coordinate medial and lateral eye rotations.